This section provides background information related to the present disclosure which is not necessarily prior art.
It is known to use hot wires or thermocouples to heat seal and simultaneously trim portions of film together. The film may be various types of thermoplastic films. Typically, two film portions are guided through a pair of pressure nippers so that a wire or thermocouple is positioned across a path of the two film portions causing the wire to seal the two film portions together and simultaneously separate or trim a waste of the film along the seal.
U.S. Pat. No. 7,281,362, entitled FILM SIDE SEALING APPARATUS WITH CLOSED-LOOP TEMPERATURE CONTROL OF A HEATER, and assigned to Shrink Packaging Systems Corporation, is an example of such a known apparatus and is incorporated by reference. This patent teaches the use of tight temperature control of a heating element using a closed loop system.
The inventors have discovered that such tight temperature control is unnecessary. In fact heat sealing and trimming may reliably be accomplished by pulsing power to the heating element in an open loop manner (i.e. without any feedback control signals), based only on the speed of the film passing the heating element. No accounting for the heating element temperature is needed to provide stable operation over extended time periods without any resin build-up. Described below is an energy efficient, open loop control that applies varying power levels to a heating element based on at least of speed of the film passing the heating element.
Open loop power pulsing for creating lap seals is known, as shown in FIGS. 1A and 1B. However, an important distinction between a lap seal and the present disclosure of a seal and trim apparatus is with respect to the trim and the nature of the seal formed. In creating a lap seal, two film portions 10 and 12 are overlapped as shown. Drive mechanisms, not shown, cause the two film portions to move in the direction of arrow 14 over drag wire 16. The film portions 10 and 12 are only to be sealed together, as indicated by cross-hatching at 18. There is no film trimming required or desired in forming a lap seal. In fact, the drag wire 16 is typically raised and lowered, as indicated at arrow 20. The drag wire 16 is typically lowered out of contact with the film portions to protect against drag wire 16 burning through the film portions 10 and 12 when the film is not moving. Further, drag wire 16 requires less power than required for a sealer and trimmer because the drag wire only seals two films portions together as opposed to sealing and trimming two film portions.
Prior to this disclosure no one appreciated that an open loop power control scheme could effectively and efficiently be applied to a seal and trim apparatus. The prior art, as discussed in U.S. Pat. No. 7,281,362 cited above, believed that a closed loop control for holding tight tolerances on the temperature of a heating element was needed to provide a robust reliable heat seal and trim device; or, if an open loop control was used, an unstable system would result in frequent stoppage of the machine for removing resin buildup on the heating element and the need for frequent operator adjustments to the power feed. As shown by the examples below, open loop control of power to the heating element can result in effective and efficient sealing and trimming of film portions by basing the power supplied on one or more of the speed of the film portions, the gauge of the film portions without controlling the temperature of the heating element.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.